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Aluminum melting in a reverberatory furnace.

Back to the Basics of Molten-Metal Management

Jan. 13, 2023
Managers must take responsibility for metal quality by taking steps to ensure workers have the principles and resources to deliver clean, hydrogen-free, inclusion-free aluminum, on time and at temperature.

In my 43 years of traveling all over the world one thing has become increasingly apparent. Some diecasters and foundries have lost the basic understanding of molten-metal management. In the past three years, working as a consultant, I have been asked to talk to companies about this subject more than in all the years I worked in furnace manufacturing and supply.

If you ask 10 different melt shop managers what molten-metal management is, you may get 10 different answers. Molten-metal management is “managing workers’ ability to deliver clean, hydrogen-free, inclusion-free aluminum to the casting equipment on time and at temperature.” Pretty straight forward, isn’t it? But amazingly this concept has been displaced in the effort to increase production and reduce labor requirements, and – because of a lack of qualified workers – acceptance of poorer quality workmanship. Buyers are demanding higher- and higher-quality castings, and so rejects are up and scrap rates are high, and this trend gets worse when foundries are overwhelmed with work, as all of them have been for the last few years.

It all starts with leadership. Someone must take ownership of metal quality. There are people being put in melt-shop management positions with little or no experience, and being told to do things the way we always have done them. There is usually a better way. About 40% of my time was spent training companies how to clean metal, clean furnaces so the linings last 5-7 years, and molten-metal management. What is the most important detail about casting aluminum parts? Is it not the metal quality, metal temperature, and on-time delivery to the holding furnaces? In the case of steel foundries, it is more likely the temperature and correct alloy.

For the last of these there is a simple fix. How can you expect molten-metal handlers to deliver metal on time if no one knows how much metal is needed per-hour, per-machine or casting line? When there is no daily plan then metal gets delivered too often or not often enough, and both lead to problems. Melt shop managers need to know how many pounds of aluminum or steel is required for each machine or line. It may vary, day to day, depending on downtime and part weights being cast. Better utilization rates for melters, holders, and people result from this basic knowledge.

This is my point: If you have radiant burners in the roof or electric elements in the roof of a holder or even a high headroom side fired melter molten-metal management is critical to their operation and efficiency. For example, if you draw your melters down 6-8 in. before refilling them, chances are they are no longer at the specified set-point temperature. Furnaces must work much harder to get the heat to the aluminum. The same thing to radiant roof holders. If you draw them down 5-6” before refilling them then chances are they will start to lose temperature.

According to the Stefan–Boltzmann Law of Radiant Heat, “the total energy radiated per unit surface area of a black body across all wavelengths per unit time (also known as the blackbody radiant emittance or radiant existence), is directly proportional to the fourth power of the black body’s thermodynamic temperature.” That is, the greater the temperature difference between your heat source and the media (aluminum) when there is a slight oxide film, the faster will be the transfer of BTUs into that load to the fourth power. Also, the closer they are to each other enhances that transfer. So, by limiting the draw down to a couple of inches, you can make furnaces run more efficiently and guarantee better temperature control. The best practice is, whatever metal you take out of a furnace in 15-20 minutes, replace with the same amount to bring it back up full again.

This is especially true with crucible furnaces. Although it may have less effect on efficiencies, it will shorten the bowl life. Crucible furnaces have extremely hot surfaces as heat rises up against the steel plate on top; when 5-6 in. of metal is removed from the bowl it is replaced with air, which is an insulator.

Electric Crucible Furnace

Now you have a build-up of heat at the top and nowhere to transfer it. There is a temperature gradient from the middle of the bowl to the top, which will crack the top of the bowl. Simply charging this crucible furnace evenly, so that the molten metal never is less than a couple of inches, may increase bowl life substantially.

Establishing a molten metal and scrap and ingot delivery system to the furnaces will help manage furnace tenders’ time and show where people are needed. In larger operations that deliver scrap to the melter by forklift, as well as ingots or sows, it is critical to get this schedule right. This should make clear when there will be time to clean the metal and furnace linings.

Constant relining is one of the most expensive items in a foundry or diecasting operation budget. Some spend hundreds of thousands of dollars on refractory relines but will not hire one more person to clean the furnace every day – which is necessary for aluminum melting furnaces. The aluminum may need to be cleaned more often than that, depending upon the alloy and ratio of scrap to new metal. The holding furnace should be cleaned every other day (except for the electric-immersion type, which can go about 5 days between cleanings.)

Molten-metal launder systems should be skimmed every day and the bottom lining scraped every week. If a build-up occurs in these systems, an overflow may result.

Pay melt shop workers on an incentive program. Track refractory costs and dross metal losses, and reward them for every dollar saved from annual average melt loss and refractory relining. You may end up with the cleanest metal and longest lasting furnaces in the business.

Run energy audits on all furnaces so you’ll know your casting temperature baselines and can tell when the refractory has been compromised. Cold-clean every furnace annually, draining them and having qualified refractory contractors remove any oxides, and patch them. Gunning over bad refractory is the worst type of repair, and hot gunning is an even greater risk. Gunning any refractory produces a final product that does not have the same properties as a cast lining. It is sometimes much lower properties than is desired.

I am always amazed at how much aluminum is sent out with the dross. Why would you want to have someone else remelt metal you have already melted and then sell it back to you? Someone told me they make money off the aluminum then sell in their dross. Even if you don’t buy it back from the dross-recovery contractor, you have to replace that metal – which is much more expensive than the income from the dross recovery. It never saves you money.

Finally, you should consider a SCADA control system for your metalcasting operation. This allows you to collect data from every casting cell and can control that cell so that it cannot produce out-of-specification parts. You can measure various parts of the casting process and record them, and if one of them is out designed parameters it will stop the cell from producing until the issue is resolved. Everything can be displayed on screen at the production manager’s desk.

In summary:
1. Start a molten-metal management program with your melt shop foreman. Try it for one month and you will see measurable results.
2. Train employees in the best practices for the specific operation.
3. Clean melters every day and holders as detailed here. Clean metal as needed.
4. Pay melt shop workers on an incentive program!
5. Work toward delivering clean metal on-time and at-temperature every day.
6. Control refractory costs through energy audits and yearly cold cleans, and if you do this, buy premium refractories. If lining life is extended by one year they will have more than paid for themselves.
7. Reduce the amount of aluminum in your dross. Do not pay twice to melt metal.
8. “You cannot control what you cannot measure” (as Peter Drucker famously said.) Implement a SCADA Control System. “Management is doing things right; leadership is doing the right things,” Drucker also said.

These few basic simple things will positively affect your bottom line.

David W. White is a consulting specialist in metalcasting technologies.